Numerous patents and literature describe techniques for removing sulfur compounds from gas stream containing the same. By far the most common technique is that used to treat natural gas having one or more of the acid gases, H.sub.2 S, SO.sub.2, COS and CO.sub.2 with an aqueous liquid lean (with respect to the acid gases) absorbent to produce a rich absorbent stream and regenerate the rich absorbent stream by thermal treatment to produce a recycleable lean stream. Many compounds have been suggested and used as the absorbent, some to selectively remove H.sub.2 S or CO.sub.2 and others more general in nature to remove as much of each of the acid gases present as is possible. Now, with the renewed interest in coal fired boilers and the like coupled with the greater concern for the environment, there is a need to provide a low pressure, at or below atmospheric, low temperature selective process to remove sulfur dioxide from the flue gases emitted from such plants without removal of any major portion of the carbon dioxide. One process for removing the SO.sub.2 widely in use today is the old and well documented limestone scrubbing process. The disadvantage of this process is that there is produced a large volume of solid waste, calcium sulfite-sulfate, often contaminated with fly ash, which must be disposed of. In certain areas of the country, that is in those areas which have paper pulp operations being carried out, the waste is oftentimes usable, but such situations are not widespread.
Another system recently in the forefront is the use of potassium or sodium citrate as disclosed and taught in U.S. Pat. No. 4,366,134. While the absorbent is regenerated and recycled, the make-up costs can be high due to thermally stable salts being formed. In addition, it has been found to be necessary to employ stainless steel for the entire plant to prevent excessive corrosion of the metals.
It would be advantageous to have a process which selectively absorbs sulfur dioxide to the almost exclusion of the other acid gases, particularly carbon dioxide, which has low chemical make-up cost, reduced operating costs and permits economical construction of equipment to process low pressure, high volume, gas streams, such as flue gas, to reduce or eliminate the sulfur dioxide content of such gases.